While
the number of confirmed extrasolar planets is now approaching 300, the tally of
extrasolar moons so far identified is still a rather disappointing zero.
Planets
beyond our solar system are incredibly challenging to find. Moons are nearly
impossible with today's technology, given that they are generally expected to
be quite small compared to their parent worlds.
Even
Earth's moon is invisible on the famous "pale
blue dot" image obtained by Voyager 1 from the comparatively small
distance of 3.7 billion miles — a photograph taken from well within our solar
system.
But
the search is not impossible, says Darren Williams, associate professor of
physics and astronomy at Penn State Erie, the Behrend College. Williams believes
a moon in orbit around a known extrasolar planet will also be detectable if we
look hard enough with the right techniques.
"It
will add a periodic component to the combined infrared signal" of the
planet-moon system, he said.
Why it matters
Finding
moons is more than just an academic quest to count them up. Planetary
satellites can be highly interesting in their own right.
It's
possible, for example, that life
could exist on extrasolar moons, researchers say.
And it has been suggested that the
ocean tides induced
by Earth's moon may have been necessary to create the conditions
for life on our planet to begin. At the least, the evolution of life has
been affected by our moon's constant tugging.
"We certainly owe our present
climate stability to the Moon and its stabilizing influence on the spin axis, but I'm not convinced that big moons are a requirement for simple or advanced life," Williams said. "I do think that Earth would have evolved advance life
even with greater seasonal extremes, but it may have taken a different evolutionary path."
How to find them
Williams
has modelled an Earth-like planet with moons of varying sizes and concluded
that satellites as small as Earth's moon could be detectable in the infrared
data, owing to their large surface temperature variations. By studying an
extrasolar planet and building up a picture of that world's infrared output,
any sizable moons present should be detectable in this way.
So
far, however, no planet as small as Earth has been detected around another
star. But astronomers expect that barrier to be broken soon. Future missions,
such as NASA's Terrestrial Planet Finder and The European Space Agency's Darwin,
will have the ability to return the valuable data required both for finding
other Earths and, Williams figures, some moons.
"The
present goal is to build instruments capable of seeing something as large as
the Earth or possibly Mars. Smaller Mercury- or Titan-sized objects fall below
that first-order threshold," Williams said.
So
could these missions cut to the chase and spot an extrasolar moon directly?
"They
might, if the light collectors are big enough and if the moons are big enough.
It will be easier to see moons that happen to transit the face of a star, such
as what the space telescope Kepler
will attempt to do starting next year," Williams explained. The space-based
Kepler observatory will note dips in starlight caused by planets crossing in
front of stars. If the planets are aligned in such a favourable manner, then
thinking goes, moons ought to transit the stars too.
A
similar conclusion is reached by Szabó, Szatmáry, Diveki and Simon in a paper
published in Astronomy and Astrophysics in 2005. They conclude that the
Kepler mission should identify a few extrasolar moons using this method of
detection.
Upon reflection
Yet
even if we are not lucky enough to catch an extrasolar moon in transit, these
future space-based planet hunters will be able to do the observational
groundwork, in visible light and in the infrared, needed to search for
satellites.
These
planet finders will even be capable of detecting the glint of starlight
reflecting off any oceans of liquid water an extrasolar planet may harbor.
"Water
is extremely dark in the infrared except when the light reflects from the
surface at a glancing angle," Williams told SPACE.com.
This
glint will be most apparent when the planet is in a crescent phase, when the
starlight hits the reflective surface at an oblique angle. (Mercury and Venus,
as seen from Earth, go through phases similar to our moon. Observations of
other planets around distant stars will undergo phasing, too.) Observing such
reflections can help map the planet's thermal output and infer the distribution
of oceans and continents.
Indeed
the Mars
Express spacecraft is set to observe crescent Earth's ocean reflection this
summer and in fall of 2009 to help understand the phenomenon.
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